Human hematopoietic progenitor cell preparations and their expansion in a liquid medium

ABSTRACT

A process for supporting hematopoietic progenitor cells in a culture medium which contains at least one cytokine effective for supporting the cells and preferably, is essentially free of stromal cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No. 07/682,344 filed Apr. 9, 1991.

INTRODUCTION

[0002] 1. Technical Field

[0003] The field of this invention is growth and expansion of hematopoietic cells in culture.

[0004] 2. Background

[0005] Mammalian hematopoiesis has been studied in vitro through the use of various long-term marrow culture systems. Dexter and co-workers described a murine system from which CFU-S and CFU-GM could be assayed for several months, with erythroid and megakaryocytic precursors appearing for a more limited time. Maintenance of these cultures was dependent on the formation of an adherent stromal cell layer composed of endothelial cells, adipocytes, reticular cells, and macrophages. These methods were soon adapted for the study of human bone marrow. Human long-term culture systems were reported to generate assayable hematopoietic progenitor cells for 8 or 9 wks and, later, for up to 20 wks. Such cultures are again relying on the preestablishment of a stromal cell layer which is frequently reinoculated with a large, heterogeneous population of marrow cells. Hematopoietic stem cells have been shown to home and adhere to this adherent cell multilayer before generating and releasing more committed progenitor cells. Stromal cells are thought to provide not only a physical matrix on which stem cells reside, but also to produce membrane-contact signals and/or hematopoietic growth factors necessary for stem cell proliferation and differentiation. This heterogeneous mixture of cells comprising the adherent cell layer presents an inherently complex system from which the isolation of discrete variables affecting stem cell growth has proven difficult.

[0006] Recently, a study was conducted by McNiece and Langley which examined the stimulatory effect of recombinant human stem cell factor (MGF) on human bone marrow cells alone and in combination with recombinant human colony stimulating factors, GM-CSF, IL-3 and erythropoietin. The results showed that MGF stimulation of low density non-adherent, antibody depleted CD34+ cells suggests that MGF directly stimulates progenitor cells capable of myeloid and erythroid differentiation.

[0007] 3. Relevant Literature

[0008] Conditions which allow long term in vitro bone marrow culture are described in Dexter, et al. (1977) J. Cell. Physiol. 91:335-344; Gartner, et al. (1980) P.N.A.S. 77:4756-4759 and Hocking, et al. (1980) Blood 56:118-124. Survival of granulocytic progenitors is shown by Slovick, et al. (1984) Exp. Hematol. 12:327-338. Roberts, et al. (1987) J. Cell. Physiol. 132:203-214 describes the use of 3T3 cells in such cultures.

[0009] Cell surface antigen expression in hematopoiesis is discussed in Strauss, et al. (1983) Blood 61:1222-1231 and Sieff, et al. (1982) Blood 60:703-713. Descriptions of pluripotential hematopoietic cells are found in McNiece, et aL (1989) Blood 74:609-612 and Moore, et al. (1979) Blood Cells 5:297-311. Characterization of a human hematopoietic progenitor cell capable of forming blast cell containing colonies in vitro is found in Gordon, et al. (1987) J. Cell. Physiol. 130:150-156 and Brandt, et al. (1988) J. Clin. Invest. 82:1017-1027.

[0010] Characterization of stromal cells is found in Tsai, et al. (1986) Blood 67:1418-1426 and Li,et al. (1985) Nature 316:633-636. The localization of progenitor cells in the adherent layer of cultures is discussed in Coulombel, et al. (1983) Blood 62:291-297 and Gordon, et al. (1985) Exp. Hematol. 13:937-940.

[0011] Eliason, et al. (1988) Exp. Hematol. 16:307-312 describes GM-CSF and IL-3 in hematopoiesis. The effect of growth factors in megakaryopoiesis is found in Brno, et al. (1988) Exp. Hematol. 16:371-377. McNiece, et al (1991) Exp. Hematol. 19:226-231 describes the use of stem cell factor in in vitro cultures.

SUMMARY OF THE INVENTION

[0012] In accordance with an aspect of the present invention there is provided a process for supporting mammalian bone marrow cells wherein such cells are maintained in a culture medium essentially free of stromal cells and which includes at least one cytokine effective for supporting such cells.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0013] Preferred embodiments of this aspect of the present invention provide a process for supporting bone marrow cells which are hematopoietic stem cells, a process for supporting bone marrow cells which are hematopoietic progenitor cells and a process for supporting bone marrow cells which are CD34+DR−CD15-cells.

[0014] In addition this invention provides that at least one cytokine be selected from the following cytokines: Interleukin (IL)-1, IL-3, IL-6, granulocyte/macrophage-colony stimulating factor (GM-CSF), human or murine stem cell factor, sometime referred to as human or murine mast cell growth factor (MGF) or Steel Factor (SL) and a fusion protein of GM-CSF/L3 (FP).

[0015] In accordance with another aspect of the present invention there is provided a process for supporting mammalian bone marrow cells wherein such cells are maintained in a culture medium containing a combination of cytokines effective for supporting such cells. Preferably, the bone marrow will be supported in a culture medium which is essentially free of stromal cells.

[0016] Additional preferred embodiments of this invention provide a process for supporting bone marrow cells which are hematopoietic stem cells. A process for supporting bone marrow cells which are hematopoietic progenitor cells and a process for supporting bone marrow cells which are CD34+DR−CD15- cells.

[0017] Preferably, the culture medium will contain at least one of the following cytokine combinations IL-1/IL-3; IL-3/IL-6; IL-3/MGF; IL-3/GM-CSF; MGF/FP. Applicant has found that such combinations provide for an improved rapid expansion of the cell population.

[0018] The term “supporting” with respect to stem cells and other progenitor cells means maintaining and/or expanding and/or promoting some differentiation of such cells.

[0019] The following are representative examples of cytokines which may be employed in the present invention. The cytokines may be human in origin, or may be derived from other mammalian species when active on human cells. IL-1 is used in an amount effective to support the cells, generally such amount is at least 1 U/ml and need not exceed 10 U/ml, preferably 2.5 U/ml, where the specific activity is 10⁸ CFU/mg protein. IL-6 is used in an amount effective to support the cells, generally such amount is at least 500 pg/ml and need not exceed 10 ng/ml, preferably 1 ng/ml. IL-3 is used in an amount effective to support the cells, generally such amount is at least 500 pg/ml and need not exceed 2 ng/ml, preferably 500 pg/ml. GM-CSF is used in an amount effective to support the cells, generally such amount is at least 100 pg/ml and need not exceed 1 ng/ml, preferably 200 pg/ml. c-kit ligand (MGF, steel factor, stem cell factor) may be human or murine in origin, is used in an amount effective to support the cells, generally such amount is at least 10 ng/ml and need not exceed 500 ng/ml, preferably 50 to 100 ng/ml. FP (fusion protein of IL-3 and GM-CSF as described in Broxmeyer, et al. [1990] Exp. Hematol. 18:615) is used in an amount effective to support the cells, generally such amount is at least I ng/mi and need not exceed 25 ng/ml, preferably 10 ng/ml.

[0020] The use of a cytokine in the absence of stromal cells is particularly suitable for expanding the mammalian bone marrow stem cells and in particular progenitor cells. The cells which are supported in accordance with the present invention are preferably of human origin.

[0021] In accordance with a preferred aspect of the present invention, a cell population which is supported in accordance with the present invention is that which is positive for CD34 antigen and is negative for HLA-DR and is also negative for CD15.

[0022] Specifically this aspect of the present invention provides for a cell population of CD34+DR- CD15- supported in accordance with the process described above, where the population has doubled in a period of time which does not exceed 15 days. Preferably, the population has doubled in 7 to 15 days.

[0023] In accordance with another aspect, the present invention provides for a cell population of bone marrow cells supported in accordance with the process described herein, where the population has doubled in a period of time which does not exceed 15 days. Preferably, the population has doubled in 7 to 15 days.

[0024] In accordance with another aspect, the present invention provides for a cell population of hematopoietic progenitor cells supported in accordance with the process described herein, where the population has doubled in a period of time which does not exceed 15 days. Preferably, the population has doubled in 7 to 15 days.

[0025] In accordance with another aspect, the present invention provides for a cell population of hematopoietic progenitor cells supported in accordance with the process described herein, where the population has doubled in a period of time which does not exceed 15 days. Preferably, the population has doubled in 7 to 15 days.

[0026] Another aspect of the present invention provides for a composition comprised of an expanded bone marrow cell culture which is essentially free of stromal cells, the culture also contains at least one cytokine and the cultures cell population has doubled in a time not exceeding 15 days. Preferably, the cell population will have doubled in at least 7 and not exceeding 15 days.

[0027] An additional aspect of the present invention provides for a composition comprised of an expanded bone marrow cell culture which contains a combination of cytokines and the cultures cell population has doubled in a time not to exceed 15 days. Preferably, the cell population has doubled in at least 7 and not exceeding 15 days. It is also preferable, that the cell culture be essentially free of stromal cells.

[0028] As previously indicated, the present invention is particularly applicable to marrow cells that are positive for CD34 antigen but which do not express HLA-DR, CD15 antigens in that it is believed that such cell population is believed to be closely associated with human hematopoietic stem cells, but it is to be understood that the present invention is not limited to supporting such a cell population.

[0029] The cells supported in accordance with the present invention may be used in a variety of ways. For example, such cells may be employed as part of a bone marrow transfer procedure.

[0030] Expanded hematopoietic stem cell populations can be used as grafts for hematopoietic cell transplantation. Autologous transplantation may be used to treat malignancies, while heterologous, transplantation may be used to treat bone marrow failure states and congenital metabolic, immunologic and hematological disorders. Marrow samples will be taken from patients with cancer prior to cytoreductive therapy, and CD34+DR−CD15-cells isolated by means of density centrifugation, counterflow centrifugal elutriation, monoclonal antibody labelling and fluorescence activated cell sorting. The stem cells in this cell population will then be expanded ex vivo and will serve as a graft for autologous marrow transplantation. The cells will be expanded in accordance with the methods of the subject invention, culturing the cells with a combination of growth factors as described. The cellular preparation will be administered to the patient following curative cytoreductive therapy e.g. chemo-radiotherapy.

[0031] Expanded stem cell populations an also be utilized for in utero transplantation during the first trimester of pregnancy. Fetuses with metabolic and hematologic disorders will be diagnosed prenatally. Marrow will be obtained from normal individuals and CD+DR-CD15- cells will be obtained by the methods described previously and expanded in vitro. They will then be administered to the fetus by in utero injection. A chimera will be formed which will lead to partial but clinically significant alleviation of the clinical abnormality.

[0032] The invention will be further described with respect to the following examples; however, the scope of the invention is not to be limited thereby.

[0033] The following examples are offered by way of illustration and not by way of limitation.

EXPERIMENTAL EXAMPLE 1

[0034] A. Materials and Procedures

[0035] Prior to performing any procedures, informed consent was obtained from all volunteers according to the guidelines of the Human Investigation Committee of the Indiana University School of Medicine.

[0036] Cell separation techniques. Bone marrow aspirates were collected from the posterior iliac crests of normal volunteers. Low-density mononuclear bone marrow (LDBM) cells were obtained by density centrifugation of the heparinized marrow over Ficoll-Hypaque (Pharmacia Fine Chemicals, Piscataway, N.J.) at 500 g for 25 min. LDBM cells were suspended in PBS-EDTA (PBS, pH 7.4, containing 5% FBS, 0.01% EDTA wt/vol, and 1.0 g/liter D-glucose) and injected into an elutriator system at 10° C. at a rotor speed of 1,950 rpm using a JA-17 rotor and standard separation chamber (Beckman Instruments, Inc., Palo Alto, Calif.). A fraction of the LDBM eluted at a flow rate of 12-14 mi/min (FR 12-14), enriched for hematopoietic precursors, was collected as described in Brandt, et al. (1988) J. Clin. Invest 82:1017-1027.

[0037] Long-term marrow cultures free of stromal cells. Plastic 35-mm tissue culture dishes were seeded with 2×10⁶ LDBM cells in 1 ml of Iscove's with 10% FBS and 2×10⁻⁵ M methylprednisolone. Cultures were incubated at 37° C. in 100% humidified atmosphere containing 5% CO₂ in air and fed weekly by total replacement of media. Stromal cells were confluent by 4-6 wk. The stromal cultures were then irradiated with 1,500 rad, the media were replaced, and the cultures were inoculated with 5×10³ sorted bone marrow cells from autologous donors. The media in these cultures were removed at 7-10 d intervals and replaced with fresh media. Suspended, nonadherent cells were then counted and assayed for progenitors.

[0038] Long-term suspension cultures. Plastic 35-mm tissue culture dishes containing 1 ml of Iscove's with 10% FBS were inoculated with stromal cell free long-term marrow cells containing 5×10³ cells obtained by sorting and incubated at 37° C. in 100% humidified atmosphere containing 5% CO₂ in air. At this time, and every 48 h thereafter, cultures received nothing (1% BSA/PBS), 2.5 U/ml IL-1α, 50 U/ml IL-3, 75 U/ml IL-6, 12.5 U/ml GM-CSF, or combinations of the above. At 7 d intervals, cultures were demidepopulated by removal of one-half the culture volume which was replaced with fresh media. Cells in the harvested media were counted, transferred to slides for staining and morphological examination, and assayed for various progenitor cells.

[0039] Hematopoietic growth factors. All cytokines were obtained from the Genzyme Corp., Boston, Mass. Recombinant IL-1α and IL-3 each had a specific activity of 10⁸ CFU/mg protein, while that of IL-6 was 10⁷ and granulocyte/macrophage colony-stimulating factor (GM-CSF) 5×10⁷ CFCc/mg protein.

[0040] Two- and three-color cell sorting. FR 12-14 cells were incubated with mouse monoclonal anti-HPCA-1 (CD34) of the IgG₁ subclass (Becton Dickinson Immunocytometry Systems, San Jose, Calif.), washed, and stained with Texas red-conjugated, subclass-specific goat anti-mouse IgGl (Southern Biotechnology Associates, Inc., Birmingham, Ala.). Cells were next incubated with mouse serum to block any unbound active sites on the second-step antibody. Cells were finally stained with phycoerythrin-conjugated mouse anti-HLA-DR either alone or in combination with FITC-conjugated CD33 (My9, Coulter Immunology, Hialeah, Fla.), CD15 (Leu-M1), or CD71 (transferrin receptor) (Becton Dickinson Immunocytometry Systems). CD15 is present on cells of the granulocytic and monocytic lineages, and an anti-CD15 monoclonal antibody was employed in the hope of eliminating these cellular components from the cell populations (Strauss, et al. [1983] Blood 61:1222-1231). CD71 is present on actively proliferating cells and an anti-CD71 antibody was utilized to separate actively proliferating cells from more quiescent marrow elements (Sieff, et al. [1982] Blood 60:703-713). Controls consisted of the corresponding isotype-matched, nonspecific myeloma proteins used in parallel with staining monoclonal antibodies. Cells were stained at a concentration of 2×10^(7/)ml and washed after each step in 1% BSA in PBS. A temperature of 4° C. was maintained throughout the procedure.

[0041] Immediately after staining, cells were sorted on a Coulter Epics 753 dual-laser flow cytometry system (Coulter Electronics, Inc., Hialeah, Fla.). Texas Red was excited by 590 nm light emitted from a rhodamine 6G dye laser. FITC and phycoerythrin were excited using the 488 nm wavelength from a dedicated 6-W argon laser. Sorting windows were first established for forward angle light scatter (FALS) and Texas red fluorescence. Positivity for each fluorochrome was defined as fluorescence >99% of that of the controls. Cells were next gated on the presence or absence of detectable HLA-DR-phycoerythrin and CD33-FITC, CD15-FITC, or CD71-FITC.

[0042] Hematopoietic progenitor cells assays. Cells were suspended at various concentrations in 35-mm plastic tissue culture dishes (Costar Data Packaging, Cambridge, Mass.) containing 1 ml of 30% FBS, 5×10⁻⁵ M 2-mercaptoethanol, 1 U human purified erythropoietin (50 U/mg protein, Toyobo Co. Ltd., Osaka, Japan), 50 U GM-CSF, and 1.1% methylcellulose in Iscove's modified Dulbecco's medium. The cultures were incubated at 37° C. in a 100% humidified atmosphere containing 5% CO₂ in air. After 14 d, erythropoietic bursts (BFU-E), granulocyte-macrophage (CFU-GM), and miXed lineage (CFU-GEMM) colonies were scored in situ on an inverted microscope using standard criteria for their identification (Brandt, supra).

[0043] High proliferative potential colony-forming cell (HPP-CFC)-derived colonies were enumerated after 28 d in culture according to the recently published criteria of McNiece and co-workers. The human HPP-CFC-derived colony is a late-appearing, very large (0.5 mm or more in diameter) colony composed primarily of granulocytes with a lesser number of monocytes; cell numbers frequently exceed 50,000.

[0044] Cells removed from suspension cultures were assayed for CFU-megakaryocyte (CFU-MK) colonies using the serum-depleted method described in detail by Bruno et al. (1988) Exp. Hematol 16:371-377. 5×10³ cells per point were suspended in a 1-ml serum-substituted fibrin clot with 100 U of IL-3 in 35-mm culture dishes and incubated at 37° C. in a 100% humidified atmosphere containing 5% CO₂ in air. At 18-24 d, cultures were fixed in situ and stained using rabbit anti-human platelet glycoprotein antisera, and fluorescein-conjugated goat F(ab′)₂-specific anti-rabbit IgG Clago, Inc., Burlingame, Calif.) and megakaryocyte colonies were enumerated on a Zeiss fluorescence microscope (Carl Zeiss, Inc., New York, N.Y.). A positive colony was defined as a cluster of three or more fluorescent cells.

[0045] B. Experiments

[0046] A liquid culture system supplemented with repeated 48-hourly cytokine additions was utilized to study cell populations. Total cell production by both CD34⁺DR⁻CD15⁻ and CD34⁺DR⁻CD71⁻ cells is shown in Tables I and II while assayable CFU-GM in these cultures over time are recorded in Tables III and IV. In the absence of exogenous cytokines, total cell numbers declined over a 2-week period and assayable CFU-GM persisted for only 1 or 2 wk. The repeated addition of IL-1α did not significantly enhance total cell production or generation of CFU-GM by either CD34⁺DR⁻CD15⁻ or CD34⁺DR⁻CD71⁻ cells. IL-6 did not alter total cell numbers or numbers of assayable CFU-GM in cultures initiated with CD34⁺DR⁻CD71⁻ cells. By contrast, IL-6 increased total cell numbers over seven fold by week 3 by CD34⁺DR⁻CD15⁻ initiated cultures but did not appreciably extend the interval over which CFU-GM were detected. In both sets of experiments, GM-CSF promoted increased total cell production for 6 wk, by which time cell numbers represented 20-80 times the number present in the initial seeding populations. Assayable CFU-GM persisted for 3-4 weeks and cumulatively surpassed those assayable in the initial populations. The single most effective cytokine in terms of promoting cellular expansion, increasing the number of CFU-GM, and lengthening the duration of time over which CFU-GM were assayable was IL-3. Both CD34⁺DR⁻CD15⁻ and CD34⁺DR⁻CD71⁻ cells experienced 200-fold increases in cell numbers by day 28, and, after 1 or 2 weeks in culture, contained equal or slightly greater numbers of CFU-GM than present in the initial inoculi. Assayable progenitors were produced for 4-5 weeks in the system when maintained with IL-3, and viable cell counts remained high at 8 wk. IL-1α or IL-6 prolonged and enhanced these effects when added in combination with IL-3. CFU-GM were assayable after 8 weeks in suspension culture after continued treatment with these two cytokine combinations. No adherent cell layer was established in any of the suspension cultures over the 8-week period of observation.

[0047] In a separate experiment, CD34+DR⁻CD71⁻ cells were grown in this suspension culture system in the presence of a combination of both IL-3 and IL-6 and assayed for CFU-MK from days 7 through 28 of culture. CFU-MK were detected over this 28 d period (Table V). Utilizing this IL-3/IL-6 cytokine combination, the ability of CD34⁺DR⁻CD15⁺ and CD34⁺DR⁻CD71⁺ cells to sustain long-term hematopoiesis was compared to that of the CD34⁺DR⁻CD15⁻ and CD34⁺DR⁻CD71⁻ fractions (Table VI). Both the CD15-positive and CD71-positive cells failed to generate CFU-GM after 2 wk, and the CD71-positive population, which initially included the overwhelming majority of BFU-E, failed to produce assayable BFU-E after only 7 d in culture.

[0048] Morphological analysis of the cells in these suspension cultures during the period of observation revealed changes in the cellular composition of the populations following the addition of various cytokines (Tables VII and VII). IL-1α-and IL-6-containing cultures behaved very similarly to the control samples. Cultures to which no cytokihes were added were composed of 90-100% blasts after 1 wk; the CD34⁺DR⁻CD15⁻ cells did not survive 2 weeks in the absence of cytokine whereas the CD34⁺DR⁻CD71⁻ initiated cultures were composed of 40% blasts and 60% monocytes by week 2. Cultures receiving IL-1α had a similar cellular composition. IL-6 facilitated some differentiation to the granulocytic series by both cell populations; the CD34+DR⁷CD15⁻ cells produced a significant number of mature granulocytic elements by week 2. GM-CSF, as well as IL-3, reduced the percentage of blasts in these suspension cultures appreciably by day 7. GM-CSF-containing cultures of CD34⁺DR⁻CD15⁻ and CD34⁺DR⁻CD71⁻ cells consisted primarily of metamyelocytes through 4 wk, with a shift to monocytes occurring by week 6.

[0049] IL-3 was unique in that, at 3 wk, suspension cultures initiated by either CD34⁺DR⁻CD15⁻ or CD34⁺DR⁻CD71⁻ cells were composed of 48% basophils in the presence of this growth factor (Tables VII and VIII). Addition of IL-1α or IL-6 did not alter this trend, all IL-3-containing cultures being composed of about 50% basophils by 3 weeks and retaining significant numbers of basophils throughout the duration of culture.

[0050] The cellular composition of hematopoietic colonies assayed from aliquots of the suspension cultures was comparable to those assayed form the original sorted populations with a few notable exceptions. Blast cell colonies, as well as HPP-CFC-derived colonies, were routinely obtained by directly assaying CD34⁺DR⁻CD15⁻ or CD34^(+DR) ⁻CD71⁻ cells while these colony types were not observed in subsequent clonal assays of cellular aliquots obtained from the long-term liquid cultures. Distribution of GM colony subtypes, however, remained fairly consistent with roughly 40% being granulocyte macrophage, 40% monocyte macrophage, and 20% basophil or eosinophil colonies in either assays initiated with sorted cells of those initiated on days 7 through 42 of liquid culture. These CFU-GM-derived colonies ranged in size from 100 to 2,000 cells with the average colony containing between 200 to 400 cells. After 8 weeks of suspension culture, monocyte macrophage colonies were the predominant colony type observed in the clonal assays. TABLE I Total Cell Production of CD34+, DR⁻, CD15⁻ Cells after Addition of Various Cytokines Day Cytokine 0 7 14 21 28 35 42 56 viable cell count × 10³ None 5  1  4  0  0 0 0 0 IL-1* 5  2  2  0  0 0 0 0 IL-3⁺ 5 53 140 591 1,085   533  678  781  IL-6^(§) 5  3  4  36  26 16  0 0 GM-CSF^(°) 5  8  14  44 169 213  118  0 IL-1α/IL-3 5 32 167 556 1,360   1,387    758  1,069    IL-6/IL-3 5 47 171 471 854 1,440    1,200    1,216   

[0051] TABLE II Total Cell production of CD34⁺, DR⁻, CD71⁻ Cells after Addition of Various Cytokines Day Cytokine 0 7 14 21 28 35 42 56 viable cell count × 10³ None 5 1 2 0 0 0 0 0 IL-1* 5 3 0 0 0 0 0 0 IL-3⁺ 5 40 226 964 746 1,190 1,120 851 IL-6^(§) 5 1 2 0 0 0 0 0 GM-CSF° 5 3 34 44 45 445 438 0 IL-1α/IL-3 5 23 202 684 1,112 835 800 1,067

[0052] TABLE III Total CFU-GM Production by CD34+, DR⁻, CD15⁻Cells after Addition of Various Cytokines Week Cytokine 1 2 3 4 5 6 7 CFU-GM/ml culture None 40 0 0 0 0 0 0 IL-1* 22 14 0 0 0 0 0 IL-3⁺ 432 696 591 325 0 0 0 IL-6§ 42 242 96 0 0 0 0 GM-CSF° 273 200 219 0 0 0 0 IL-1α/IL-3 254 397 444 408 139 152 64 IL-6/IL-3 98 342 236 768 864 1,080 384

[0053] TABLE IV Total CFU-GM Production by CD34+, DR⁻, CD71⁻ Cells after Addition of Various Cytokines Week Cytokine 1 2 3 4 5 6 8 CFU-GM/ml culture None 15 4 0 0 0 0 0 IL-1a* 20 0 0 0 0 0 IL-3⁺ 664 272 96 448 119 0 0 IL-6^(§) 51 14 0 0 0 0 0 GM-CSF° 402 360 135 28 0 0 0 IL-1/IL-3 347 324 342 334 167 240 214

[0054] TABLE V Assayable CFU-MK in Long-Term Suspension Cultures of CD34+DR⁻CD71⁻ Cells Receiving a Combination of IL-3 and IL-6 Days in culture* CFU-MK/ml culture+  7 42.6 ± 7.6§ 14 67.6 ± 56.6 21 17.0 ± 11.8 28 20.2 ± 10.4

[0055] TABLE VI Total CFU-GM and BFU-E Production by Sorted Cell Populations Stimulated with a Combination of IL-3 and IL-6 Week Population 1 2 3 4 6 8 CFU-GM (BFU-E)ml culture CD34+DR⁻CD15⁻ 275(10) 286(4) 64 32 75 0 CD34+DR⁻CD15⁺  7(1)  26  0  0  0 0 CD34+DR⁻CD71⁻ 220(5) 330(4) 132  18 43 0 CD34+DR⁻CD71⁺  13  16  0  0  0 0

[0056] TABLE VII Differential Analysis of CD34⁺, DR⁻, CD15⁻ Cells after Addition of Various Cytokines Cytokine Day Blasts Pro Myelo MM Band % Seg Eo Baso E Mo Control 7 100 IL-1a* 7 100 14 78 22 1L-6⁺ 7 100 14 27 11 9 13 38 2 21 9 48 2 7 17 17 28 30 4 66 GM-CSF^(§) 7 25 24 27 3 21 14 9 1 46 3 21 13 7 21 3 2 1 62 3 5 22 2 28 6 1 43 7 3 6 2 32 35 4 96 42 1 99 IL-3° 7 21 44 35 1 14 7 7 53 33 21 8 44 48 28 5 35 3 9 35 13 35 2 16 5 20 25 32 42 15 2 20 63 IL-1α 7 1 5 1 53 12 14 14 /IL-3 14 5 34 9 52 21 1 53 4 3 31 8 28 1 42 12 5 32 8 35 20 27 53 42 8 8 84 56 11 89 IL-6/IL-3 7 19 26 2 40 5 4 4 14 2 2 46 3 1 46 21 5 1 37 1 7 48 1 28 4 1 37 10 8 35 5 42 1 8 1 9 81 56 2 3 95

[0057] TABLE VIII Differential Analysis of CD34+, DR⁻, CD71⁻ Cells after Addition of Various Cytokines Cytokine Day Blasts Pro Myelo MM Band % Seg Eo Baso E Mo Control 7 90 10 14 40 60 IL-1α* 7 82 18 IL-6⁺ 7 43 4 13 14 33 20 47 GM-CSF^(§) 7 39 33 9 5 6 5 2 14 18 5 42 3 12 20 21 4 1 66 9 7 4 28 2 61 3 1 8 24 35 14 18 8 8 9 52 42 100 1L-3° 7 52 40 1 2 2 2 1 14 29 26 26 2 3 14 21 13 4 2 28 2 3 48 28 14 3 35 5 1 35 7 35 9 20 7 6 27 31 42 2 5 4 16 2 71 IL-1α/ 7 48 42 6 2 1 2 IL-3 14 4 1 53 4 5 33 21 3 44 1 1 49 2 28 21 3 34 4 3 1 27 8 35 3 23 4 29 20 21 42 1 7 3 3 16 70 56 1 8 91

EXAMPLE 2

[0058] Long-term bone marrow cultures (LTBMC) were initiated with 5×10³ CD34⁺DR⁻CD15⁻ marrow cells/ml in plastic 35-mm tissue culture dishes containing 1 ml of Iscove's with 10% PBS and incubated at 37° C. in 100% humidified atmosphere containing 5% CO₂ in air. At this time, and every 48 h thereafter, cultures received nothing, or murine mast cell growth factor (MGF; c-kit ligand) alone or in combination with IL-3 or a GM-CSF/IL-3 fusion protein (FP; Broxmeyer, et al. Exp. Hematol. 18: 615, 1990). In cultures not receiving cytokines, viable cells were not detectable after two weeks while cultures receiving IL-3, FP, or MGF sustained hematopoiesis for 10 weeks. Addition of IL-3 or FP alone increased cell numbers by 100 fold by day 26, while the combination of MGF and FP expanded cell numbers 1000-fold (5×10³ cells at day 0; 12,500×10³ at day 26). Over the 10 week period of LTBMC, treatment with various cytokines led to the following cumulative increases over an input of 213 total assayable hematopoietic progenitor cells (HPC; CFU-GM+BFU-E+CFU-MK): IL-3, 868: FP, 1,265; MGF, 2,006; MGF+IL-3, 4,845; MGF+FP, 155,442. LTBMCs receiving MGF alone possessed a higher HPC cloning efficiency than those receiving IL-3 or FP and its addition increased the cloning efficiencies of cultures containing of IL-3 and FP. The presence of MGF did not increase the longevity of cultures receiving these cytokines. TABLE IX Total Cell Production of CD34+, DR⁻, CD15⁻ Cells after Cytokine Addition Day Cytokine 0 26 Viable. Cell count × 10³ None 5 0 *IL-3 5 140 ^(+GM-CSF) 5 100 °FP 5 1400 MGF 5 520 GM-CSF/IL-3 5 560 MGF/ 5 12,500 GM-CSF MGF/IL-3 5 1,200 MGF/FP 5 10,000

[0059] TABLE X Differential Analysis of CD34+, DR⁻, CD15⁻ Cells After Addition of Various Cytokines on Day 26 of Suspension Culture Cytokine Blasts Pro Myelo MM Band Seg Lymph Eo Baso Mo Norm FP 3 7 9 9 27 3 5 2 9 0 5 GM-CSF/ 1 7 4 13 24 32 4 3 4 0 0 IL-3 MGF 32 4 9 9 13 12 7 1 1 12 0 MGF/ 21 10 15 12 14 7 5 2 3 11 0 GM-CSF MGF/IL-3 38 3 15 12 13 4 2 2 4 7 2 MGF/FP 37 17 16 9 9 5 1 0 6 0 5

EXAMPLE 3

[0060] Liquid culture systems supplemented with repeated 48-hourly cytokine additions were utilized to study cell populations cultured from two donors. Total cell production of CD34⁺DR⁻CD15⁻ cells is shown in Table XI while assayable CFU-GM in these cultures over time is recorded in Table XIH. In the absence of exogenous cytokines, total cell numbers declined over a 1 to 2-week period and assayable CFU-GM persisted for only a 1 to 2-week period. In donor 1, MGF/FP cytokine combination promoted increased total cell production for 8 weeks, by which time cell numbers represented over 110×10³ times the number present in the initial seeding populations. In donor 2 the same cytkine combination promoted increased total cell production for 6 weeks, by which time the cell numbers represented by over 16×10³ times the number present in the initial seeding population. Assayable CFU-GM for donor 1 and donor 2 cultured with MGF/FP cytokine combination persisted for 6-8 weeks and 3-4 weeks, respectively and significantly surpassed the CFU-GM population initially assayable.

[0061] The cytokine combination MGF/IL-3 promoted over 2×10³ fold increase in total cell production over the initial seeding for donor 1 at 6 weeks and donor 2 at 8 weeks. Additionally, viable cell counts remain high through 10 weeks. The assayable expansion of CFU-GM for donor 1 and 2 cultured with MGF/IL-3 cytokine combination persisted for 6-8 weeks for each donor and each significantly surpassed the CFU-GM population assayable initially.

[0062] Total BFU-E production by CD34⁺DR⁻CD15⁻ cells is shown in Table XIV. In donor 1 and donor 2 the cytokine combination MGF/FP persisted for 1-2 weeks and 3-4 weeks, respectively with only donor 2 showing a significant increase over the BFU-E population initially assayable. The cytokine combination MGF/IL-3 persisted in donor 1 for 2-3 weeks and in donor 2 for 3-4 weeks, with both showing significant increase in weeks 1-2 over the BFU-E population initially assayable.

[0063] Total CFU-MK production by CD34+DR⁻CD15⁻ cells is shown in Table XV. The cytokine combination of MGF/IL-3 for both donors 1 and 2 show CFU-MK persistence for through 10 weeks and each has significantly surpassed the initially assayable CFU-MK population. Donors 1 and 2 show CFU-MK persistence for 6-8 weeks and 8-10 weeks, respectively, both showing significant increases over the initial CFU-MK population.

[0064] Morphological analysis of the cells in the suspension cultures of donor 1 during the period of observation revealed changes in the cellular composition of the population following the addition of various cytokines, see Table XII, which shows the differential analysis of CD34⁺DR⁻CD15⁻ cells. Cultures receiving MGF/FP were composed of 11% blasts by 14 days and cultures receiving MGF/IL-3 were composed of 17% blasts by 14 days. The highest percentage of blasts by 14 days was in the cultures receiving MGF alone which were composed of 30% blasts. In contrast IL-3 and FP containing cultures had reduced the percentage of blasts cells appreciably by day 14.

[0065] Table XVI depicts the percentage of total cells which give rise to progenitor cells of colony forming units. Although MGF percentages are high the overall expansion of cultures receiving MGF is not as substantial, however the cultures receiving MGF/IL-3 cytokines provide high plating percentages and substantial overall expansion (see Tables XI-XV). TABLE XI Total Cell Production of CD34+DR⁻CD15⁻ Cells Cultured in the Presence of Various Cytokines viable cell count × 10³/ml Cytokine Week 1 2 3 4 6 8 10 Donor 1 None 1 0 0 0 0 0 0 IL-3¹ 28 144 271 560 480 762 960 GM-CSF² 12 107 436 1,085 2,680 2,080 1,760 IL-3/ 23 244 742 1,620 1,979 2,035 2,720 GM-CSF FP³ 42 262 587 1,240 3,000 1,494 480 MGF⁴ 8 104 933 N.D.⁵ 1,680 1,760 640 MGF/FP 101 1,211 35,100 101,000 262,400 550,000 100,000 MGF/ 38 213 978 2,820 10,800 5,680 5,120 IL-3 Donor 2 None 1 0 0 0 0 0 0 IL-3 24 180 650 605 1,400 960 864 FP 41 810 2,100 6,680 1,840 4,320 5,280 MGF 8 27 71 98 230 70 0 MGF/FP 100 1,280 15,700 6,400 81,000 19,520 0 MGF/ 36 305 780 1,380 6,960 10,400 5,440 IL-3 Donor 3 MGF/FP N.D. 5,040 14,400 14,800 8,960

[0066] TABLE XII Differential Analysis of CD34+DR⁻CD15⁻ Cells Following Culture with Various Cytokines Cytokine Day Blast % Pro Myelo Meta Band Seg Baso Eos Mono Post Sort 0 82 1 1 6 10 IL-3¹ 7 10 8 16 2 9 50 2 3 14 2 4 39 4 3 10 28 10 28 3 6 13 3 1 6 61 7 FP² 7 10 21 52 5 2 7 3 14 1 4 17 8 3 20 14 33 28 1 24 7 4 36 8 20 MGF³ 7 54 39 3 1 3 14 30 38 9 1 1 1 20 28 1 7 21 18 13 15 1 1 23 MGF/FP 7 29 22 23 3 4 18 1 14 11 22 16 4 2 4 13 28 28 1 8 13 12 2 10 2 52 MGF/ 7 31 15 48 2 4 IL-3 14 17 14 9 2 4 8 34 12 28 8 46 17 2 17 2 8

[0067] TABLE XIII Total CFU-GM Production by CD34+DR⁻CD15⁻ Cells Cultured in the Presence of Various Cytokines CFU-GM/ml culture¹ Cytokine Week 1 2 3 4 6 8 Donor 1 None 8 0 0 0 0 0 IL-3² 132 28 80 N.D.⁵ N.D. 128 GM-CSF³ 192 112 88 N.D. 128 0 IL-3/ 196 104 36 N.D. 128 576 GM-CSF FP⁴ 86 112 128 176 N.D. 64 MGF⁵ 290 396 608 448 96 0 MGF/FP 376 1,600 14,800 58,000 80,000 0 MGF/IL-3 144 348 104 416 2,528 192 Donor 2 None 0 0 0 0 0 N.D. IL-3 232 196 96 16 64 ND. FP 84 148 288 320 544 N.D. MGF 106 152 360 64 128 N.D. MGF/FP 114 1,440 10,600 N.D. N.D. N.D. MGF/IL-3 62 240 504 32 1,024 N.D. Donor 3 MGF/FP N.D. 12,448 32,936 32,264 1,254 0

[0068] TABLE XIV Total BFU-E Production by CD34⁺ DR⁻ CD15⁻ Cells Cultured in the Presence of Various Cytokines BFU-E/ml culture¹ Cytokine WEEK 1 2 3 4 Donor 1 None 0 — — — IL-3 24 0 0 0 GM-CSF 8 0 0 0 IL-3/ GM-CSF 22 4 0 0 FP 20 4 0 0 MGF 8 40 0 0 MGF/FP 98 0 0 0 MGF/IL-3 238 4 0 0 Donor 2 Control 0 — — — IL-3 40 28 0 0 FP 132 68 56 16  MGF 6 0 0 0 MGF/FP 662 100 200 0 MGF/IL-3 1,062 272 40 0

[0069] TABLE XV Total CFU-MK Production by CD34⁺ DR⁻ CD15⁻ Cells Cultured in the Presence of Various Cytokines CFU-MK/ml culture¹ Cytokine Week 2 3 4 6 8 10 Donor 1 None 0 0 0 0 0 0 IL-3² 14 74 100 118 48 N.D.⁵ GM-CSF³ 12 40 20 48 32 0 IL-3/GM-CSF 20 80 96 120 N.D. N.D. FP⁴ 28 120 184 118 96 64 MGF⁵ 6 12 36 20 0 0 MGF/FP 40 120 120 120 N.D. 0 MGF/IL-3 26 90 208 220 128 64 Donor 2 None 8 0 0 0 0 0 IL-3 26 100 140 140 64 64 GM-CSF 24 40 60 80 32 0 IL-3/GM-CSF 40 120 160 200 64 64 FP 56 120 200 200 96 64 MGF 10 36 60 60 0 0 MGF/FP 56 200 200 200 40 0 MGF/IL-3 34 120 240 260 160 192

[0070] TABLE XVI Plating Efficiency of CD34⁺ DR⁻ CD15⁻ Cells Cultured in the Presence of Various Cytokines Cytokine Week 1 2 3 4 6 8 % Plating Efficiency¹ None N.D.⁶ — — — — — IL-3² 0.86 0.072 0.023 0.003 0.005 0.009 GM-CSF³ 1.67 0.105 0.020 N.D. 0.005 0.000 IL-3/GM-CSF 0.96 0.044 0.005 N.D. 0.006 0.028 FP⁴ 0.42 0.039 0.019 0.010 0.015 0.002 MGF⁵ 2.58 0.493 0.580 0.065 0.031 0.000 MGF/FP 0.65 0.127 0.056 0.029 0.015 0.000 MGF/IL-3 2.10 0.173 0.041 0.008 0.019 0.002

EXAMPLE 4

[0071] Serum-free long-term suspension human bone marrow culture system. Serum free liquid culture system media was prepared as described by Ponting, et al. (1991) Growth Factors 4:165-173. Both serum free and serum containing cultures were initiated with CD34⁺DR⁻CD15⁻ cells, and supplemented every 48 hours with c-kit ligand (MGF) and a GM-CSF/IL-3 fusion protein (FP).

[0072] As shown in Table XVII, cultures maintained in serum-free media were characterized by far less total cell production than has been observed in comparable serum containing cultures. Over six weeks of observation, these cultures only increased total cell number by 24-fold. However, the progenitor cell cloning efficiency in serum-free cultures was 1.4% after 28 days of LTBMC, in comparison to a cloning efficiency of 0.03% in comparable serum-containing cultures. These studies suggest that the serum-free culture system is preferable for expanding progenitor cell numbers at the expense of impairing the production of more differentiated cells. TABLE XVII Plating Efficiency of CD34⁺ DR⁻ CD15⁻ Cells Cultured in Serum-Free Medium* Progenitor Cells Days in Culture Cell no. × 10³ CFU-GM HPP-CFC  0 10 375 40 14 30 744  9 28 70 1,050   21 42 140  140 42

[0073] It is evident from the above results, that substantial cell expansions may be obtained from hematopoietic progentor cells in the substantial or complete absence of stromal cells. These cultures may be grown and maintained for extended periods of time, where progenitor cells are able to expand and maintain the culture. Disadvantages associated with the presence of stromal cells, the lack of a controlled system dependent upon the nature and activity of the stromal cells, is avoided by using factors individually or in combination. In this manner, substantial expansions of hematopoietic cells from a limited number of progenitors can be achieved in a controlled substantially reproducible manner.

[0074] All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference.

[0075] Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. 

What is claimed is:
 1. A process for supporting mammalian bone marrow cells in a culture medium, which comprises: maintaining bone marrow cells in a culture medium which is essentially free of stromal cells, said culture medium containing at least one cytokine effective for supporting said cells.
 2. A process as in claim 1 , wherein said bone marrow cells are hematopoietic stem cells.
 3. A process as in claim 1 , wherein said bone marrow cells are hematopoietic progenitor cells.
 4. A process as in claim 1 , wherein said bone marrow cells are CD34⁺DR⁻CD15⁻ cells.
 5. A process as in claim 1 , wherein at least one said cytokine is selected form the group consisting of IL-1; IL-3; IL-6; MGF; Fusion Protein of GM-CSF/IL-3.
 6. A process for supporting manmmalian bone marrow cells in a culture medium, which comprises: maintaining bone marrow cells in a culture medium which contains a combination of cytokines effective for supporting said cells.
 7. A method acording to claim 6 , wherein said culture medium is essentially free of stromal cells.
 8. A process as in claim 6 , wherein said bone marrow cells are hematopoietic stem cells.
 9. A process as in claim 6 , wherein said bone marrow cells are hematopoietic progenitor cells.
 10. A process as in claim 6 , wherein said bone marrow cells are CD34⁺DR⁻CD15⁻ cells.
 11. A process as in claim 6 , wherein at least one said cytokine is selected form the group consisting of IL-1; IL-3; IL-6; MGF; Fusion Protein of GM-CSF/IL3. 